For example, Patent literature 1 discloses a technique of performing friction stir welding on the inner corner portion at which a pair of metal members are abutted vertically against each other. This technique teaches a rotation tool having a base block in a triangular prism shape and a stirring pin which penetrates the base block for joining a pair of metal members which form an inner corner by abutting the pair of metal members each other. In this technique the friction stir welding is performed by bringing each slope of the base block into contact with each of the pair of metal members respectively and by rotating the stirring pin at high speed while pushing the stirring pin into an inner corner portion.